Density of insulation

Contacting plates, steel, including typical liquid loading, 1.2kN/m plate area. Typical values for the density of insulating materials are (all kg/m ) ... 

THE DETERMINATION OF THE THERMAL CONDUCTIVITY OF THE SPECIFIC HEAT AND THE DENSITY /GROSS DENSITY/ OF INSULANTS FOR ROCKET TANKS FILLED WITH LIQUID HYDROGEN. PARTI. DESCRIPTION OF MEASURING METHOD. 

Density of insulating brick based on this data will be found to be less than... 

Some nonhygroscopic materials such as metals, glass, and plastics, have the abiUty to capture water molecules within microscopic surface crevices, thus forming an invisible, noncontinuous surface film. The density of the film increases as the relative humidity increases. Thus, relative humidity must be held below the critical point at which metals may etch or at which the electrical resistance of insulating materials is significantly decreased. 

For low frequency electrical insulation appHcations, the dielectric constant of the insulation is ideaUy as low as possible (see Insulation, electrical). The lower the density of the ceUular polymer, the lower the dielectric constant and the better the electrical insulation. Dielectric strength is also reduced at... 

Thermal Conductivity and Aging. Thernial performance is governed by gas conduction and radiation (18—20). In most ceUular plastic insulations, radiation is reduced because normal densities of use ate 4-50 kg/m and the average cell size is <0.5 mm. For open-ceU and other materials containing air (at 24°C, 7 = 0.025 W/(m-K)) this results in total values of X at 0.029-0.0039 W/(m-K). 

Foam. PhenoHc resin foam is a cured system composed of open and closed ceUs with an overall density of 16—800 g/cm. Principal appHcations are in the areas of insulation and sponge-like floral foam. The resins are aqueous resoles cataly2ed by NaOH at a formaldehyde phenol ratio of ca 2 1. Free phenol and formaldehyde content should be low, although urea may be used as a formaldehyde scavenger. 

Mechanically Fastened. The substrate, ie, insulation, cover board, etc, which the single-ply membrane must go over, is typically mechanically fastened to the deck using a low density of fasteners. However, there are also appHcations where the membrane is appUed directly to the deck. The typical... 

Insulation having high thermal endurance has been made from vitreous siUca fibers (see Ablative materials). Such material forms the basis for the 30,000 iasulatiag tiles, 125-mm thick, that protect the aluminum skin of the space shuffle. The tiles add a minimum of weight because the density of the iasulation is only 144 kg/m (9 lb/fT), similar to balsa wood. 

However, conductive elastomers have only ca <10 of the conductivity of soHd metals. Also, the contact resistance of elastomers changes with time when they are compressed. Therefore, elastomers are not used where significant currents must be carried or when low or stable resistance is required. Typical apphcations, which require a high density of contacts and easy disassembly for servicing, include connection between Hquid crystal display panels (see Liquid crystals) and between printed circuit boards in watches. Another type of elastomeric contact has a nonconducting silicone mbber core around which is wrapped metalized contacts that are separated from each other by insulating areas (25). A newer material has closely spaced strings of small spherical metal particles in contact, or fine soHd wires, which are oriented in the elastomer so that electrical conduction occurs only in the Z direction (26). 

The abihty to accept and hold the electrostatic charge in the darkness. The photoconductive layer should support a surface charge density of approximately 0.5-2 x 10 C/cm. The charge also has to be uniformly distributed along the surface, otherwise nonuniformities can print out as spot defects. The appHed surface potential should be retained on the photoreceptor until the time when the latent electrostatic image is developed and transferred to paper or, if needed, to an intermediate belt or dmm. In other words, the "dark decay" or conductivity in the dark must be very low. The photoconductor materials must be insulators in the dark. 

Multilayer Insulation Miiltilayer insulation consists of alternating layers of highly reflec ting material, such as aluminum foil or aluminized Mylar, and a low-conduc tivity spacer material or insulator, such as fiberglass mat or paper, glass fabric, or nylon net, all under high vacuum. When properly applied at the optimum density, this type of insulation can have an apparent thermal conduc tivity as low as 10 to 50 jlW/m-K between 20 and 300 K. 

Foam Insulation Since foams are not homogeneous materials, their apparent thermal conductivity is dependent upon the bulk density of tne insulation, the gas used to foam the insulation, and the mean temperature of the insulation. Heat conduction through a foam is determined by convection and radiation within the cells and by conduction in the solid structure. Evacuation of a foam is effective in reducing its thermal conductivity, indicating a partially open cellular structure, but the resulting values are stiU considerably higher than either multilayer or evacuated powder insulations. 

After a few years the tank base can no longer be cathodically protected by the initial protection current density of 3 mA m Only after installing additional anodes can the tank base be again cathodically protected with protection current densities of 10 mA m" with a total protection current of 80 A. The reason for the high protection current requirement is unsatisfactory insulation of the tank foundation. 

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